This invention relates to the field of nickel titanium-based shape memory alloys and particularly to those alloys containing niobium.
Materials, both organic and metallic, capable of possessing shape memory are well known. An article made of such materials can be deformed from an original, heat-stable configuration to a second, heat-unstable configuration. The article is said to have shape memory for the reason that, upon the application of the heat alone, it can be caused to revert or attempt to revert from its heat-unstable configuration to its original heat-stable configuration, i.e., it "remembers" its original shape.
Among metallic alloys the ability to possess shape memory is a result of the fact that the alloy undergoes a reversible transformation from an austenitic state to a martensitic state with a change of temperature. Also, the alloy is considerably stronger in its austenitic state than in its martensitic state. This transformation is sometimes referred to as a thermoelastic martensitic transformation. An article made from such an alloy, for example, a hollow sleeve, is easily deformed from its original configuration to a new configuration when cooled below the temperature at which the alloy is transformed from the austenitic state to the martensitic state. The temperature at which this transformation begins is usually referred to as M.sub.s and the temperature at which it finishes M.sub.f. When an article thus deformed is warmed to the temperature at which the alloy starts to revert back to austenite, referred to as A.sub.s (A.sub.f being the temperature at which the reversion is complete), the deformed object will begin to return to its original configuration.
Commercially viable alloys of nickel and titanium have been demonstrated to have shape-memory properties which render them highly useful in a variety of applications.
Shape-memory alloys have found use in recent years in, for example, pipe couplings (such as are described in U.S. Pat. Nos. 4,035,007 and 4,198,081 to Harrison and Jervis), electrical connectors (such as are described in U.S. Pat. No. 3,740,839 to Otte and Fischer), switches (such as are described in U.S. Pat. No. 4,205,293 to Melton and Mercier), etc., the disclosures of which are incorporated herein by reference.
It is, of course, advantageous to have the alloy austenitic at the service temperature which is often but not necessarily near room temperature, since the austenite phase is stronger than the martensite phase. In fact, it would be desirable to have the alloy remain austenitic over a wide range of service temperatures, for example from substantially below room temperature to substantially above room temperature, so that the alloy has practical utility.
As an illustration, Military Specification MIL-F-85421 requires a product that is functional to about -55.degree. C. If the product comprises a shape memory alloy, then for convenience in shipping the product in the heat-unstable configuration, the product should not recover prior to about 50.degree. C. It is a matter of commercial reality, within and without the military, that the product satisfy these or similar requirements.
It is also desirable that the alloy be martensitic in the vicinity of room temperature so that the article can be fabricated, stored, and shipped at or near room temperature. The reason for this is that in the case of an article made from the alloy, a coupling, for example, the article would not recover prematurely.
Conceptually, one way to achieve these desirable results, to wit, an alloy that is martensitic near room temperature and which is also austenitic over a large range of temperatures including room temperature, is to have an alloy which exhibits a sufficiently wide tranformation hysteresis, say, greater than about 125.degree. C. If the hysteresis were sufficiently wide and room temperature could be located near the middle of the hysteresis, then the alloy could be fabricated and conveniently stored while in the martensitic condition. Since the hysteresis is sufficiently wide, the alloy would not transform to austenite until heated substantially above room temperature. This heating would not be applied until the alloy (in the form of a coupling, for example) was installed in its intended environment. The alloy, which would then be in the austenitic condition, would remain in the austenitic condition after cooling down since the service temperature (which may be above or below room temperature) would be substantially above the martensite transformation temperature. Thus, the above-noted desirable results could be achieved.
Unfortunately, there is believed to be no commercially viable nickel/titanium-based alloy that has a hysteresis sufficiently wide to achieve these desirable results.
For example, the commercially viable near equiatomic binary nickel-titanium alloys can have a hysteresis width of about 30.degree. C. The location of the hysteresis for this alloy is also extremely composition sensitive so that while the hysteresis can be shifted from sub-zero temperatures to above-zero temperatures, the width of the hysteresis does not appreciably change. Thus, if the alloy were martensitic at room temperature, the service temperature must be above room temperature. Similarly, if the service temperature was at room temperature, the alloy would be martensitic below room temperature so that the alloy would require special cold-temperature equipment for fabrication, shipping, and storage. Ideally, as discussed above, room temperature should be located near the middle of the transformation hysteresis. However, since the width of the hysteresis in the binary alloy is so narrow, the range of service temperatures for any particular alloy is necessarily limited. As a practical matter, the alloy would have to be changed to accommodate any change in service temperatures.
It can be appreciated that the relative lack of commercialization of shape memory alloys must be due, at least in part, to their extreme sensitivity to temperatures as discussed above. Alloying and processing have not solved the problem.
Nickel/titanium/iron alloys, e.g., those in Harrison et al., U.S. Pat. No. 3,753,700, while having a wide hysteresis, up to about 70.degree. C., are the typical cryogenic alloys which always undergo the martensite/austenite transformation at sub-zero temperatures. It should be noted that in general, the colder shape-memory alloys such as the cryogenic alloys have a wider transformation hysteresis than the warmer shape memory alloys. In the case of the cryogenic alloys, the alloys must be kept very cold, usually in liquid nitrogen, to avoid the transformation from martensite to austenite. This makes the use of shape memory alloys inconvenient, if not uneconomical.
The nickel/titanium/copper alloys of Harrison et al., U.S. patent application Ser. No. 537,316, filed Sept. 28, 1983, and the nickel/titanium/vanadium alloys of Quin, U.S. patent application Ser. No. 541,844, filed Oct. 14, 1983, now U.S. Pat. No. 541,844 are not cryogenic but their hysteresis may be extremely narrow (10.degree.-20.degree. C.) such that their utility is limited for couplings and similar articles.
Nickel/titanium/niobium alloys are largely unexplored. The ternary phase diagram has been determined [see "Ternary Intermetallic Compounds in the System Ni-Ti-Nb", Poroshkovaya Metallurgiya, No. 8(44), pp. 61-69 (1966)] but there has been no study of the physical properties in this system. U.S. Naval Ordinance Laboratory Report NOLTR 64-235 (August, 1965) examined the effect upon hardness of ternary additions of from 0.08 to 16 weight percent of eleven different elements (including niobium) to stoichiometric nickel/titanium.
The problems experienced with the nickel/titanium-based shape memory alloys have been somewhat overcome by processing in the copper-based shape memory alloys. It is now known that the hysteresis in copper-based shape memory alloys can be temporarily expanded by mechanical preconditioning, austenitic aging and heat treating. In this regard, see Brook et al., U.S. Pat. Nos. 4,036,669; 4,067,752; and 4,095,999.
The methods of the Brook et al. patents have been applied to nickel/titanium-based alloys; however, it has been found that these methods have no beneficial effect on nickel/titanium-based alloys.
It is known that under certain conditions the hysteresis of nickel/titanium-based alloys can be shifted as opposed to expanded. It should be understood that shifting of the hysteresis means that the M.sub.s, M.sub.f, A.sub.s, and A.sub.f temperatures have all been translated to M.sub.s ', M.sub.f ', A.sub.s ' and A.sub.f ' such that there is substantially no change in the width of the hysteresis. It should be noted that the translated transformation temperatures may be higher or lower than the normal transformation temperatures. On the other hand, expansion of the hysteresis should generally be understood to mean that A.sub.s and A.sub.f have been elevated to A.sub.s ' and A.sub.f ' while at least M.sub.s and usually also M.sub.f remain essentially constant. Aging, heat treatment, composition, and cold work can all effectively shift the hysteresis. For example, if the stress is applied to the shape memory alloy at room temperature the hysteresis may be shifted so that the martensite phase can exist at a temperature at which there would normally be austenite. Upon removal of the stress, the alloy would isothermally (or nearly isothermally) transform from martensite to austenite.
Miyazaki et al., ("Transfomation Pseudoelasticity and Deformation Behavior in a Ti-50.6 at % Ni Alloy", Scripta Metallurgica, vol. 15, no. 3, pp. 287-292, (1981) have studied the deformation behavior of binary nickel-titanium alloys. As implied in FIG. 3 of this reference, the austenite transformation temperatures can be elevated when nonrecoverable strain is imparted to the alloy. That is, when the alloy was strained to 8% or higher and the stress then removed, there was some component of the strain which remained at the deformation temperature of 243.degree. K. (compared to an A.sub.f of 221.degree. K.). This component recovered when heated to 373.degree. K. (see dotted lines on FIG. 3) although the precise recovery temperature was never measured. It is not clear from this reference whether the hysteresis was shifted or expanded since the binary nickel-rich alloy tested is extremely unstable when rapidly quenched as was done in this reference. In fact, one skilled in the art would have concluded that the hysteresis was shifted and not expanded due to the unstable alloy tested. There is no illustration of the transformation hysteresis to contradict this conclusion.
In the Melton et al. patent previously mentioned, a nickel/titanium/copper alloy was deformed beyond a critical strain so as to impart nonrecoverable strain. However, no expansion of the transformation hysteresis was reported.
While it can be appreciated that it would be desirable to have a nickel/titanium-based shape memory alloy and article with a sufficiently wide transformation hysteresis, the prior art has thus far remained silent on a way to achieve it.
Thus, it is an object of the invention to have a nickel/titanium-based shape memory alloy which is capable of having a wide transformation hysteresis.
Another problem common to nickel/titanium-based shape memory alloys is their notoriously poor machinability. Of course, while nickel/titanium-based shape memory alloys can be machined, it is only with expensive tooling and then only in relatively simple shapes.
It can be appreciated that a free-machining nickel/titanium-based shape memory alloy would be extremely desirable. Unfortunately, the prior art has also remained silent on how to achieve such an alloy.
Thus, it is another object of the invention to have a nickel/titanium-based shape memory alloy that is free-machining.
Still another problem with shape memory alloys is that many alloys transform to an "R" phase at temperatures above the normal martensite transformation temperature. The R phase is a transitional phase between austenite and martensite. Generally, in alloys with M.sub.s temperatures below -70.degree. C., the R phase becomes manifest at significantly higher temperatures. In couplings, the R phase transformation leads to a relaxation of stresses upon cooling before the M.sub.s temperature is reached.
It would be desirable to have an alloy with no deleterious R phase transformaton. That is, it is desirable to have an alloy with an R phase transformation at as low a temperature as possible or at least an R phase transformation that does no interfere with the austenite/martensite transformation. Most desirable is no R phase transformation at all.
Thus, it is a further object of the invention to have a nickel/titanium-based shape memory alloy that does not experience a deleterious R phase transformation.
While it is certainly desirable that the shape memory alloy have a wide transformation hysteresis, be free-machining and not exhibit a deleterious R phase transformation, it is important to appreciate and understand that recovery strength, ductility, and stability also remain important considerations when choosing a shape memory alloy.
Thus, it is a still further object of the invention to have a nickel/titanium-based shape memory alloy which is exemplary with respect to recovery strength, ductility, and stability.
These and other objects of the invention will become apparent to those skilled in the art after reference to the following description considered in conjunction with the accompanying drawings.